In the past few years, many micromechanical and microelectromechanical devices (hereinafter collectively referred to as "MEMs devices") that include mechanical members have been fabricated from silicon or other etchable materials. These MEMs devices are advantageous because they can be made with microfabrication techniques having increased precision, allow for smaller miniaturization, and generally have lower power requirements.
Although the production of MEMs devices having etched mechanical members has been expanding, several manufacturing problems have not yet been adequately addressed. For example, one problem is testing the MEMs devices during manufacture to ensure that the devices provide the desired operational and performance characteristics. It is typically desirous to test MEMs devices at the wafer level so that device quality can be evaluated prior to packaging. Cost and schedule advantages are achieved by wafer probe testing in identifying only good performing devices worthy of investment for assembly, and quantifying device performance at completion of fabrication (thus communicating device characteristics at completion of fabrication, unmasked by further assembly effects). Wafer probe testing requires successful probe contact of die access pads and the electrical excitation and measurement of devices.
Traditionally, MEMs devices have been tested by mechanically exciting the mechanical members and recording the electrical outputs of the MEMs devices. The electrical outputs of the MEMs devices due to the mechanical excitation indicate the level of quality and performance of the MEMs devices.
Testing MEMs devices using mechanical excitation, however, has some disadvantageous limitations. MEMs devices are typically manufactured by forming a group of MEMs devices on one single etchable die or wafer. In typical mechanical excitation procedures, however, the MEMs devices must first be separated from the wafer before they can be individually tested. The separation of the MEMs devices is time consuming, and the resulting individual MEMs devices are typically delicate, small, and thus, tedious to handle during testing. Further, the time expended separating the MEMs devices that are later found to malfunction is essentially wasted. In addition to requiring that the MEMs devices be separated from the wafer before testing, mechanical excitation procedures also require specialized machinery designed to exert several different mechanical forces on the MEMs devices.
To address the problems associated with mechanical excitation of MEMs devices, testing procedures have been implemented that use electrical excitations as opposed to mechanical excitation. These test procedures apply electrical signals to the various mechanical members of the MEMs devices. These electrical signals excite the various mechanical members causing the mechanical members to move similar to the movement caused by mechanical excitation. This movement of the mechanical members produces electrical signals at the output of the MEMs devices. By analyzing the electrical signals output by the MEMs devices, which are indicative of the movement of the mechanical members caused by electrical excitation, the level of quality and performance of the MEMs devices can be determined. Electrical excitation procedures are advantageous since they may be implemented while the MEMs devices are still part of the wafer, i.e., without requiring the MEMs devices to be individually separated from the wafer for testing. As such, time expended for removing MEMs devices that are later determined to be malfunctioning and problems associated with handling of the individual MEMs devices are eliminated if wafer probe testing is accomplished.
However, electrical excitation test procedures may also have problems which hinder efficient testing of MEMs devices. Specifically, electrical excitation of MEMs devices does not create the same level or magnitude of excitation of the mechanical members as does conventional mechanical excitation test procedures. As such, the electrical data signals output by the MEMs devices in response to electrical excitation are much smaller in amplitude. These smaller amplitude signals can make testing the MEMs devices difficult.
For instance, in a typical measurement procedure, the test apparatus uses a test probe that is connected to an output of the MEMs devices. The test probes receive the electrical data signal from the MEMs devices, and the electrical data signal propagates through the test probes, through electrical leads between the various test components, and through electrical wiring, prior to being received at a remote test station for analysis. Due to the relatively small amplitude of the electrical data signals, however, a charge amplifier must be used to amplify the signals. The charge amplifier is a specialized amplifier that converts very small charge signals (i.e., approximately 10.sup.-17 coulombs) to a voltage level sufficient for analysis.
Problems occur due to the introduction of signal noise to the electric data signal as it propagates from the test probe to the remote test station. In this regard, the test probe, component leads, and the electrical wiring on which the data signal propagates are susceptible to the introduction of electrical noise which disadvantageously decreases the signal to noise ratio. Specifically, the test probe, leads and wires are susceptible to the receipt of spurious outside noise signals such as the 60 Hz frequency of a power supply or other electrical noise such as electromagnetic and magnetoelectronic fields.
The electrical data signal output from the MEMs device has a relatively small amplitude compared to the electrical noise signals introduced into the data signal as it propagates along the test probe, component leads, and wiring. As such, the data signal deteriorates and may be obscured by the electrical noise when the electrical data signal is received by the remote test station. Specifically, when the electrical data signal is amplified at the remote location for analysis purposes, the lower amplitude electrical data signal may be obscured by the higher amplitude noise signals that are introduced into the signal as it propagates from the MEMs device to the remote test station, thereby rendering analysis of the electrical data signal virtually impossible.
Additionally, the charge amplifiers used to amplify the electrical data signals are sensitive to electrical noise such as input capacitance and electrical capacitance from spurious signal sources (e.g., 60 Hz power source). Due to the large magnitude of gain supplied by the charge amplifiers and the unique characteristics of the amplifier, the large amplitude electrical noise present on the input of the amplifier is significantly amplified. This amplification of the signal noise further causes the relatively small magnitude electrical data signal from the MEMs device to be obscured by the larger magnitude electrical noise.
While electrical stimulation of the mechanical members of a MEM's device offers numerous advantages during the testing of a MEM's device, the electrical stimulation of the mechanical members have several shortcomings. In particular, the signals received by the test probe that will be subsequently analyzed to determine the performance characteristics of the MEM's device have a relatively small amplitude which can be obscured by noise introduced and amplified during the transmission of the signals to a remote test station. As such, the signals to be analyzed must be sufficiently amplified to be large enough to present a true picture of the performance characteristics of the MEM's device since a substantial portion of the signals that are analyzed have the potential to be dominated by noise.